1. Field of the Invention
The invention relates to a stator for an electric machine and to a method for manufacturing a winding for such a stator.
2. Description of the Background Art
Electric machines are key automotive components. Electric machines may be utilized as starting motors, as alternators that generate electricity from engine motion and as electric drive motors in modern hybrid electric vehicles (HEVs).
The efficiency of an electric machine has a major effect on the fuel economy of hybrid electric vehicles and is also a major issue in other applications. Since the electric machine has to be located ‘under the hood’ of the vehicle, where space is limited, the size/dimensions of the electric machine must be as small as possible.
Most electric machines include a stator formed from a lamination stack. A plurality of electric conductors, typically in the form of copper wires, is positioned in slots of the stator.
One approach for reducing the size of the electric machine while maintaining or increasing efficiency is to increase the slot-fill ratio of the electric machine. The term “slot-fill-ratio” (SFR) is typically defined as the ratio of the aggregate cross-sectional area of the bare copper conductors in a slot to the cross-sectional area of the slot itself. With a high SFR, the large cross-sectional area of the copper wires helps reduce the phase resistance and consequently the resistance of the windings (i.e.—power loss) for a given slot size, whereby the efficiency of the machine is improved. Today, more efficient electric machines can be built at a smaller size than in the past. Armature windings of most small and mid-sized electric machines are typically wound in many turns with single or multiple strands of round wires. The SFR of the round wire electric machines can reach a maximum of 44% preventing the design of low loss (resistance), high efficiency electric machines. As discussed previously, this issue becomes even more critical when designing high efficiency machines for hybrid vehicles. The available space in onboard hybrid vehicle is strictly limited, and therefore, boosting efficiency by increasing machine size becomes impractical.
One solution for increasing the SFR is to use rectangular wires in the stator slots in place of round wires. Use of rectangular wires in the stator slots can increase the slot-fill-ratio up to 70% over that of round wires, allowing the SFR of rectangular wire machines to reach a value of near 75% or higher.
U.S. Pat. No. 6,894,417 discloses an electric machine having multi-set rectangular copper hairpin windings. The electric machine comprises a stator having a plurality of partially closed stator slots. A first winding set and a second winding set are positioned in the stator slots. The first winding set and the second winding set are interconnected by adjacent leg ends. According to a first embodiment, conductor layers in alternate slots alternate between different phases. The conductor layers in the remaining slots are all of the same phase. Hairpins having unequal length legs are used to implement the first embodiment. According to a second embodiment, conductor layers in alternate slots include one phase for the first winding set and another phase for the second winding set. The conductor layers in the remaining slots are all of the same phase. Hairpins having equal length legs are used to implement the second embodiment.